The present invention generally relates to current limiting in electrical power systems, and, more particularly to limiting fault currents in direct current and pulsed circuit systems.
Direct current electricity is used in many applications involving commercial and experimental equipment. For example, most traction systems, such as subways, streetcars, other rapid transit systems, and ship and submarine power distribution systems use direct current energy supplies. In research, direct current pulses are used in high energy and pulsed power experiments. Often, this equipment utilizes high currents to achieve a desired result.
Like all electrical systems, these direct current systems must have protective devices to shut them down in the event of a fault so that both the loads served and the direct current supply system are not damaged. Traditionally, this protection has been in the form of electro-mechanical circuit breakers. However, these circuit breakers are relatively slow, since the circuit interruption function is mechanical. This interruption time is sufficiently long as to potentially allow damage to sensitive equipment on the system.
For the design of any electrical system, it is necessary to calculate as accurately as possible the maximum current in the system, which usually occurs in a fault situation. From this information, the ratings of the electrical components, such as the wire sizes and breaker ratings are determined. However, if there were a way to limit the maximum fault current to a lower amount, the ratings of the circuit components such as wires and breakers would be lowered. Lower ratings mean that less expensive equipment could be used in the system.
The present invention provides just such a fault current limiting function, allowing direct current systems to be designed for currents only slightly above the normal load current. The invention accomplishes this through the virtually instantaneous insertion of an inductance into the fault circuit, providing a period during which the fault current is limited in value, allowing sufficient time for the separate circuit breaker to interrupt a lower fault current. This ability of the present invention allows for an overall circuit to be designed for a lower available fault current. By appropriate choice of the bias current source, the present invention can even be used to force the fault current to zero, if a fault is detected, further reducing the required equipment ratings.
It is therefore an object of the present invention to provide apparatus and method for limiting fault currents in direct current systems.
It is another object of the present invention to apparatus and method for allowing minimization of component ratings for a direct current system.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
To achieve the foregoing and other objects, and in accordance with the purposes of the present invention, as embodied and broadly described herein, a solid-state fault current limiting system for a direct current circuit including a direct current voltage source, having a source impedance, a direct current circuit breaker, a transmission line, having a line impedance including all line impedances of said direct current circuit, and a load, the solid-state current limiting system comprises a diode having a cathode and an anode, the cathode being connected to the transmission line, and the anode being connected to a first pole of said load. A controllable direct current source having a first pole is connected to the cathode of the diode. An inductor having an impedance greater with respect to the source impedance of the direct current voltage source and the line impedance of the transmission line, is connected between a second pole of the controllable direct current source and the anode of the diode. In operation, the controllable direct current source biases the diode into a conducting state until fault current from the direct current voltage source is greater in value than current flowing through the diode from the controllable direct current source, causing the diode to cease conducting, thereby routing the fault current through the controllable direct current source and the inductor and limiting rate of rise of the fault current until the circuit breaker opens.
In another aspect of the present invention and in accordance with its principles and purposes a solid-state fault current limiting system for a pulsed power load, such as a magnet, energized through a closing switch by a pulsed power source, such as a capacitor bank, having a source impedance and producing a pulse period, and a transmission line having a line impedance, the solid-state fault limiting system comprising a diode having a cathode and an anode, the cathode being connected to the closing switch through said transmission line and the anode being connected to the pulsed power load. An auxiliary pulsed current source has a first pole connected to the cathode of said diode. An inductor having an impedance greater with respect to the source impedance and the line impedance is connected between a second pole of the auxiliary pulsed current source and the anode of the diode. In operation, the auxiliary pulsed current source biases the diode into a conducting state for a time period longer than the pulse period of the pulsed power source when discharged into the pulsed power load, until fault current from the pulsed power source exceeds current from the auxiliary pulsed current source, causing the diode to cease conducting and route the fault current through the auxiliary pulsed current source and the inductor, thereby limiting rate of rise and peak value of the fault current.